When the war began, Galbraith was a student in Toronto. In 1915, accompanied by Arthur Brown and Stearne Edwards, he enrolled at the Wright School in Dayton, Ohio. After obtaining his pilot's certificate, he joined the Royal Naval Air Service and was assigned to 1 Naval Wing in France. In October 1916, having scored two victories with Nieuport scouts, he was reassigned to the newly formed 8 Naval Squadron. On 23 November 1916, he scored his final victory of the war. Flying alone in a Sopwith Pup, he engaged six German two-seaters near Cambrai, shooting one of them down. After returning to England for rest, he was an instructor and participated in anti-submarine operations in Italy during 1918. Galbraith was killed in an automobile accident three years later while serving with the Royal Canadian Air Force.

"In recognition of his services in attacking a large enemy two-seater seaplane on 28 September 1916. Flight Sub-Lieutenant Galbraith's machine was severely damaged by gun fire from the enemy machine, which finally blew up in the air." DSC citation, London Gazette, 25 October 1916

"For conspicuous gallantry. On 23 November 1916, he attacked single-handed a formation of six hostile aircraft, no other Allied machines being in the vicinity. One hostile machine was shot down, a second was driven down under control, and the remaining four machines then gave up the fight and landed. In several other combats in the air, Flight Lieutenant Galbraith has displayed exceptional gallantry, particularly on 10 and 16 November 1916, on each of which days he successfully engaged and shot down an enemy machine." DSC Bar citation, London Gazette, 16 February 1917